Everything about Ferrite Iron totally explained
Ferrite or
alpha iron (
α-Fe) is a
materials science term for
iron, or a
solid solution with iron as the main constituent, with a
body centred cubic crystal structure. It is the component which gives
steel and
cast iron their
magnetic properties, and is the classic example of a
ferromagnetic material.
It can be considered as pure
iron practically (strength = 280N/mm
2). Ferrite can be strictly defined as a solid solution of iron in body-centered cubic (
BCC) containing a maximum of 0.03% carbon at 723
oC and 0.006% carbon at room temperature.
Most "mild"
steels (plain carbon steels with up to about 0.2 wt% C) consist mostly of ferrite, with increasing amounts of
pearlite (a fine lamellar structure of ferrite and
cementite) as the carbon content is increased. Since
bainite (shown as ledeburite on the diagram) and pearlite each have ferrite as a component, any iron-carbon alloy will contain some amount of ferrite if it's allowed to reach
equilibrium at room temperature.
In pure iron, ferrite is stable below 910°C. Above this temperature the
face-centered cubic form of iron,
austenite (gamma-iron) is stable. Above 1390°C, up to the
melting point at 1539°C, the body-centred cubic crystal structure is again the more stable form of
delta-ferrite (
δ-Fe).
Only a very small amount of
carbon can be dissolved in ferrite; the maximum
solubility is about 0.02wt% at 723°C. This is because carbon dissolves in iron interstitially, with the carbon atoms being about twice the diameter of the interstitial "holes", so that each carbon atom is surrounded by a strong local
strain field. Hence the
enthalpy of mixing is positive (unfavourable), but the contribution of
entropy to the
free energy of
solution stabilises the structure for low carbon content. 723°C also is the minimum temperature at which iron-carbon austenite (0.8 wt% C) is stable; at this temperature there's a
eutectoid reaction between ferrite, austenite and
cementite.
Further Information
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